Monachine seals are a group of seals with species in the Mediterranean, North Pacific, and Antarctic oceans today. They belong to the family Phocidae, or "true seals". Well-known monachine species include elephant seals, monk seals, leopard seals, and crabeater seals. In the Southern Hemisphere today, elephant seals are the only monachine seals that breed outside the Antarctic Circle.
Most seals seen in Australia today, such as fur seals and sea lions, belong to a different family, the Otariidae or eared seals. The discovery near Portland, Victoria, of a single fossil tooth from a true seal has shown that until at least 3 million years ago, Australia was home instead to now-extinct species of monachines.
This discovery provides new information on why archaic monachines were replaced by eared seals in the deep past, in Australia and around the world. Current theories put forward by scientists suggest that this replacement was caused by the environmental changes caused by ancient episodes of global cooling.
The world was a lot warmer, and sea levels were higher, around 3 million years ago. The onset of a glacial period meant that ice formed at Earth's poles, causing global sea levels to drop. As a consequence, shallow coastal shelves became exposed, eliminating the southern beaches and prey-rich coastal waters that true seals relied upon. The new coastal habitats were rocky beaches and islands, benefiting fur seals and sea lions, whose hind flippers are better adapted to walking on land. These eared seals would have dispersed from the North Pacific to their known ranges in Australia today.
The story this fossil tells is a warning for what is to result from anthropogenically accelerated climate change. The more the Earth warms, the more polar ice will melt, leading to higher sea levels. This sea level rise will eliminate the beaches and islands that fur seals and sea lions rely upon to rest. This may lead to another restructuring of pinniped ecosystems, risking extinction for Australia's native seals.